Multimode modem with automatic negotiation of operational mode

ABSTRACT

Apparatus and methods for determining an operational mode of a far-end modem are disclosed. Where the multimode modem is a call initiator, a method according to the invention includes transmitting a V.8 ANS am  tone to the far-end modem, receiving a response signal from the far-end modem, and determining from the response signal whether the far-end modem is a commercial modem or a secure modem. Where the multimode modem is a call responder, a method according to the invention includes transmitting to the far-end modem a P1800 Hz tone with phase reversals, receiving a response signal from the far-end modem, and determining from the response signal whether the far-end modem is a commercial modem or a secure modem.

FIELD OF THE INVENTION

[0001] The present invention relates generally to modems. Moreparticularly, the present invention relates to multimode modems withautomatic negotiation of operational mode.

BACKGROUND OF THE INVENTION

[0002] Secure telephony devices use integral modems to transfer anencrypted digital bitstream representing the user's voice or other dataacross standard telephone networks. The most widely built and usedsecure telephone is the STU-III (Secure Terminal Unit, third generation)which was developed for the U.S. Government. A typical STU-III supportsdata transfers up to 9600 bps, and encodes and encrypts speech usingfederal-standard voice coders. The STU-III modem complies with a uniquesignaling plan, known as FSVS-210, which was developed for the NationalSecurity Agency.

[0003] Commercially available modems, which offer performance well inexcess of 9600 bps, are based on standards developed by theInternational Telecommunication Union (ITU), which are documented in theITU's V.series recommendations. Commercial modems complying withRecommendations V.32, V.34, and V.90 are the most prevalent.

[0004] In certain applications, it is desirable that a single device beable to communicate interchangeably with either a secure modem or acommercial modem. It is well known, however, that the modemcharacteristics specified in FSVS-210 for STU-III interoperation are notcompatible with the ITU V.series modem. Although a single device couldconceivably communicate with both STU-III and ITU V.series modems with apriori knowledge of the far-end modem's modes of operation, such asolution is impractical since the characteristics of a far end deviceusually are not known. It would be advantageous therefore, if a singledevice could communicate with both STU-III and ITU V.series modemswithout a priori knowledge of whether the far-end modem is a commercialmodem or a secure modem, or of the far-end modem's operational mode.Thus, there is a need in the art for a multimode modem that canautomatically determine and negotiate operational mode of a far-endmodem.

SUMMARY OF THE INVENTION

[0005] The present invention satisfies these needs in the art byproviding apparatus and methods for determining an operational mode of afar-end modem.

[0006] Where the multimode modem is a call initiator, a method accordingto the present invention for identifying a far-end modem type includestransmitting a V.8 ANS_(am) tone to the far-end modem, receiving aresponse signal from the far-end modem in response to the transmittedV.8 ANS_(am) tone, and determining from the response signal whether thefar-end modem is a commercial (e.g., V.series) modem or a secure (e.g.,FSVS) modem.

[0007] If the response signal is a V.8 CM tone, then the far-end modemis determined to be a V.8 modem. If, on the other hand, the responsesignal is not a V.8 CM tone, but rather, has a nominal frequency ofabout 1800 Hz, then the far-end modem can be either a V.32 modem or asecure modem.

[0008] If the response signal has a nominal frequency of about 1800 Hz,but does not include phase shifts, then the far-end modem is determinedto be a V.32 modem. If, on the other hand, the response signal includesphase shifts, then the far-end modem is determined to be a secure modem.

[0009] If the far-end modem is determined to be a secure modem, then anoperational mode of the far-end modem can be determined from theresponse signal. If the response signal includes phase reversals, thenthe far-end modem is determined to be an FSVS modem in alternate mode.If the response signal includes a 128 dibit gap, then the far-end modemis determined to be an FSVS modem in half-duplex mode. If the responsesignal does not include either phase reversals or a 128 dibit gap, thenthe far-end modem is determined to be an FSVS modem in interoperablemode.

[0010] This invention is possible since a commercial V.series modem islooking for energy at 1800 Hz. Since the transmission of an 1800 Hz tonewith 45-degree phase reversals would violate V.32 specifications (whichrequire a pure 1800 Hz tone), the V.series initiator will ignore the 45degree phase shifts present in a P1800 Hz tone, and consider the tone tobe the expected “M” tone. Also, the V.series initiator ignores the phasereversals in the “alternate mode” P1800 Hz tone, which occur in thefirst 80 msec of reply, since it looks for phase reversals only after ittransmits “AC”, which can occur no sooner than 155 msec after the startof the P1800 Hz tone.

[0011] Where the multimode modem is a call responder, a method accordingto the present invention for identifying a far-end modem type includestransmitting to the far-end modem a P1800 Hz tone with phase reversals,receiving a response signal from the far-end modem in response to thetransmitted P1800 Hz tone, and determining from the response signalwhether the far-end modem is a commercial modem or a secure modem.

[0012] An incoming channel is simultaneously monitored for the presenceof a V.8 ANS_(am) tone, and for energy at either 1800 Hz or 2100 Hz. Ifa V.8 ANS_(am) tone is present in the incoming channel, then the far-endmodem is determined to be a V.8 modem. If 1800 Hz energy is present inthe incoming channel for at least about one second, then the far-endmodem is determined to be an FSVS modem operating in half-duplex mode.If 2100 Hz energy is present in the incoming channel for at least aboutone second, then the far-end modem can be either a V.32 modem, or anFSVS modem operating in either interoperable mode or alternate mode.

[0013] If the response signal includes a V.32 AC, then the far-end modemis determined to be a V.32 modem. If, on the other hand, the responsesignal includes an FSVS Message A, then the far-end modem is determinedto be an FSVS modem in alternate signaling mode. If the response signalincludes neither a V.32 AC nor an FSVS Message A, then the far-end modemis determined to be an FSVS modem in interoperable mode.

[0014] Apparatus according to the invention include computer-readablemedia having stored thereon computer executable instructions forperforming any of the inventive methods.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing summary, as well as the following detaileddescription of the preferred embodiments, is better understood when readin conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings an embodimentthat is presently preferred, it being understood, however, that theinvention is not limited to the specific apparatus and methodsdisclosed.

[0016]FIG. 1 is a flowchart of a method for initiating a call using amultimode modem with automatic negotiation of operational mode.

[0017]FIG. 2 is a flowchart of a method for responding to a call using amultimode modem with automatic negotiation of operational mode.

[0018]FIGS. 3A-3D depict calling scenarios for an auto-negotiating modemresponding to a call from various modem initiators.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0019] STU-III Full Duplex Modem Training

[0020] A secure STU-III call typically begins from a state where bothparties are connected in a non-secure telephone conversation. Totransition the call into a secure call, one of the users activates thesecurity via their STU-III. This STU-III is commonly known as the“security initiator.” A full-duplex STU-III initiator will transmiteither a 2100 Hz echo suppressor disable (ESD) or a 2100 Hz echosuppressor canceller disable (ESCD) tone. The ESCD tone differs from theESD tone in that the ESCD tone has phase reversals every 450 msec.Whether the ESD tone or the ESCD tone if used depends on the model ofSTU-III. Older STU-IIIs typically provide a strap setting for selectingeither ESD or ESCD. Newer STU-IIIs typically do not include this strapsetting, and send only the ESD tone.

[0021] Upon detection of at least 1 second of the received 2100 Hz tone,the far-end or responding STU-III (the “security responder”) willtransmit a pseudo-1800 Hz tone (P1800). The P1800 Hz tone is not a pure1800 HZ sine wave, but rather, it is constructed by alternatelytransmitting dibits 0 and 2. This generates a tone with an averagefrequency of 1800 Hz, having alternating positive and negative phaseshifts of 45 degrees every baud-time (which is typically about {fraction(1/1200)} sec, or 0.833 msec).

[0022] The P1800 Hz tone can be generated either as an “interoperablesignaling mode” P1800 Hz tone, or as an “alternate signaling mode” P1800Hz tone. If the responding STU-III supports “alternate mode” signaling,it will transmit three 180 degree phase reversals in the P1800 Hz toneat baud times 32, 64, and 96 (26.6, 53.3, and 80 msec, respectively).For “interoperable mode”, no phase reversals are included in the P1800Hz tone.

[0023] After detecting the responder's P1800 Hz tone, the initiatingSTU-III will wait 90 msec (108 baud-times), and then terminatetransmission of the ESD or ESCD tone. If the responding STU-IIItransmitted phase reversals in the P1800 Hz tone, and the initiatingSTU-III also supports “alternate mode” signaling, the initiating STU-IIIwill transmit an FSVS “Message A” 85 msec after termination of the ESDor ESCD tone.

[0024] After detecting the initiator's removal of the ESD or ESCD tone,the responder will continue transmitting the P1800 Hz for 150 msec (180baud-times). If the start of Message A is not received by the respondingSTU-III during this time, it will transmit 4096 bits of Scrambled Ones(SCR1), followed by silence. This indicates “interoperable mode”. If thestart of Message A is received during the 150 msec, the responder willreply with an FSVS “Message B” and “alternate mode” signaling beginsusing training and modulation techniques similar to ITU V.32.

[0025] For “interoperable mode” signaling, the initiating STU-III trainsits equalizer on the received SCR1, and on completion, the initiatorsends the 2100 Hz tone again for 1 second followed by 4096 bits of SCR1,followed by silence. The responding STU-III trains its equalizer on thereceived SCR1 pattern then transmit 704 bits of SCR1 to indicate itsreadiness to exchange STU-III signaling messages. These signalingmessages and the user data are transferred using modulation techniquessimilar to ITU-V.26 standards.

[0026] Half-duplex STU-III calls can be established by the initiator'stransmitting an 1800 Hz tone instead of the 2100 Hz ESD/ESCD. Also, aresponding STU-III can force a half-duplex mode by placing gaps in itsP1800 Hz response.

[0027] V.32 Modem Training.

[0028] V.32 modems can use V.8 or V.25 startup to initiate modemtraining. The V.8 process is described below for V.34/V.90 modems. V.32modems using V.25 startup initiate modem training by transmitting a 2100Hz tone with phase reversals. This tone is transmitted from theanswer-mode modem, per the V.25 standard, and is referred to as the“ANS” tone. Note that when ITU-V.series modems are used in a securetelephony application, the security initiator is the “answer-mode” modemas defined in the ITU standards.

[0029] Upon detecting the ANS tone, the calling mode modem replies tothe ANS tone, after one second, with an “M” pattern. This pattern is afixed, 1800 Hz sinusoid, without phase shifts or reversals.

[0030] After receiving at least 100 msec of the “AA” pattern, theanswer-mode modem terminates transmission of the ANS tone, waits 75msec, and transmits the “AC” pattern, which is either a 600 Hz or a 3000Hz signal, for at least 64 baud times (2400 baud/sec), after which itswitches to transmitting the “CA” pattern. This switch from “AC” to “CA”results in a 180 degree phase reversal in the answer-mode transmittedsignal.

[0031] Upon detecting the “AC” to “CA” phase reversal, the calling-modemodem delays 64 baud-times, and changes from transmitting “AA” to “CC”.This results in 180-degree phase reversal in the 1800 Hz sinusoid.

[0032] Upon detecting The “AA” to “CC” phase reversal, the answer-modemodem waits 64 baud-times, then switches to transmitting the “AC”pattern again resulting in another phase reversal.

[0033] The calling mode modem, upon detecting the “CA” to “AC” phasereversal, terminates transmission of the “CC” pattern. The answer-modemodem detects the end of the received “CC” pattern, terminatingtransmission of “AC”, and then begins echo cancellation training.

[0034] V.34/V.90 Modem Training

[0035] V.34 and V.90 modems always employ V.8 signaling to start aconnection. Like V.25, V.8 starts with a 2100 Hz answer tone. In V.8signaling, however, this answer tone is amplitude modulated at a +/−20%level by a 15 Hz signal, and is therefore referred to as “ANS_(am)”. AV.8-compliant DCE, upon detecting the modulation of the answer tone,will proceed with V.8 signaling, whereby modem capabilities arenegotiated before modem training using a 300 bps V.21-based exchange.The allowed modem families that can be negotiated with V.8 include V.22,V.23, V.26, V.27, V.32, V.34, and V.90. After the common modemcapabilities are determined, the modem training process begins. If aV.8-compatible modem receives the V.25 ANS tone (with no amplitudemodulation), it will transition to V.32/V.32bis mode.

[0036] Automatic Modem Negotiation

[0037] A method according to the invention will now be described for anauto-negotiating modem that can determine which signaling data linklayer modem (STU-III 2400, STU-III 4800/9600, standard V.32, orV.34/V.90) to engage based on the results of modem training. Twoscenarios are described: the auto-negotiating modem as the initiator(answer-mode modem), and the auto-negotiating modem as the responder(call-mode modem).

[0038] Security Initiator

[0039]FIG. 1 is a flowchart of a method 100 for initiating a call usinga multimode modem with automatic negotiation of operational mode. Atstep 102, the auto-negotiating multi-mode modem initiates full duplexmodem training by transmitting an ANS_(am) tone in accordance with V.8for an answer-mode modem. The initiator waits, at step 104, for aresponse signal from the responder by starting a 3.3 second timer, inaccordance with the V.25 specification, and monitoring the environment,at step 106, for the presence of either of energy at 1800 Hz or a V.8“CM” pattern.

[0040] If, at step 108, the initiator receives the V.8 “CM” pattern,then the far-end modem is identified as being V.8-compatible. Ifdesirable, modem negotiation can then proceed, at step 116, using theV.8 protocol. The initiator replies to the response signal by sending aV.8 “JM” pattern to the far-end modem. Modem training in accordance withthe negotiated modem mode is completed, and the appropriate securitysignaling is performed.

[0041] If, at step 110, the initiator detects the presence of energy at1800 Hz, the initiator identifies the far-end modem as either a securemodem (STU-III), or a V.32 commercial modem. To determine which of thesethe far-end modem is, additional characteristics of the response signalare determined.

[0042] At step 118, the initiator determines whether the received 1800Hz response signal is a V.32 “AA” by looking for phase shifts in theresponse signal. If the response signal has no phase shifts, then, afterreceiving 100 msec of the “AA” pattern, the initiator proceeds, at step126, with V.32 signaling by transmitting the “AC” pattern to theresponder. The initiator continues with V.32 signaling and training, anduses the appropriate security signaling to complete the secureconnection.

[0043] If, at step 118, the initiator determines that the 1800 Hzresponse signal does include phase shifts of about 45 degrees, then theinitiator determines that the response signal is P1800 Hz, and that theresponder is a STU-III. If the initiating auto-negotiating multi-modemodem receives a STU-III P1800 Hz tone in response to the ANS_(am) toneit will proceed with STU-III signaling.

[0044] To determine the operating mode of the far-end STU-III, theinitiator first determines, at step 120, the whether the response signalincludes phase reversals. If, at step 120, the initiator determines thatthe response signal is a P1800 Hz signal with phase reversals, then theinitiator determines that the responder is a STU-III in alternate mode.At step 128, the initiator completes STU-III alternate mode signaling byterminating the ANS_(am) tone (which appears to the far-end STU-III tobe ESCD) 90 msec after detecting P1800 and then transmitting Message Aafter 85 msec. The initiator then waits for Message B from theresponder.

[0045] If, at step 120, the initiator determines that the P1800 Hzsignal does not includes phase reversals, then the initiator determines,at step 122, whether the far-end STU-III is in half-duplex mode orinteroperable mode by determining whether a 128 dibit gap is present inthe P1800 Hz tone. If the dibit gap is not present, then the initiatordetermines that the responder is a STU-III in interoperable mode and, atstep 124, completes interoperable mode signaling and then waits for SCR1from the responder.

[0046] If, at step 122, the initiator determines that there is a 128dibit gap in the P1800 Hz tone, then the initiator determines that theresponder is a STU-III operating in half-duplex mode and, at step 130,completes half-duplex signaling.

[0047] If, at step 112, the initiating auto-negotiating multi-mode modemreceives neither a V.8 “CM” nor energy at 1800 HZ in response to thetransmitted ANS_(am) tone within approximately 3.3 seconds, it willtimeout and, at step 114, indicate a secure connection failure.

[0048] An auto-negotiating multi-mode modem, when initiating half-duplexcalls, will follow the STU-III half duplex initiation protocol byoutputting a P1800 signal instead of a 2100 Hz ESCD tone.

[0049] Modem Responder

[0050]FIG. 2 is a flowchart of a method 200 for responding to a callusing a multimode modem with automatic negotiation of operational mode.While in a non-secure call, the auto-negotiating multi-mode modemcontinuously monitors for two signals from the telephone line to whichit is coupled: a 2100 Hz ESD/ESCD/V.8 tone indicating far-end modeminitiation, and a P1800 Hz tone for far-end half-duplex STU-III modeinitiation.

[0051] If, at step 202, the multimode modem determines that it hasreceived a P1800 Hz for 1024 bit times followed by a gap of 128 dibits,then the responder determines that the initiator is a STU-III inhalf-duplex mode. At step 204, the multimode modem performs STU-IIIhalf-duplex signaling in the responder role.

[0052] If, at step 206, the multimode modem determines that it hasreceived an amplitude modulated 2100 Hz tone (i.e., an “ANS_(am)” inaccordance with V.8), then the responder determines that the initiatoris a V.8 modem. At step 208, the responder replies with the V.8 “CM”sequence indicating its available modem types, and continues with modemnegotiation in accordance with V.8 (see FIG. 3A).

[0053] If, at step 210, the multimode modem determines that it hasreceived an ESD/ESCD/ANS tone (i.e., a 2100 Hz tone without amplitudemodulation) for one second, then, at step 212, the responder transmits aP1800 tone with phase reversals (the STU-III “alternate mode” signalingresponse).

[0054] While transmitting the P1800 tone, the auto-negotiatingmulti-mode modern will wait up to 150 msec after detecting a loss ofESD/ESCD/ANS to detect either a V.32 “AC” or a STU-III Message A inresponse. If the responder detects a loss of ESD/ESCD/ANS at step 214,then the responder starts a 150 msec timer at step 216.

[0055] If, at step 218, the responder detects a V.32 “AC” within the 150msec window, then the responder determines that the far-end modem is aV.32 modem. At step 220, the responder stops the 150 msec timer, andchanges the P1800 Hz transmission to the V.32 “AA.” At step 222, theresponder proceeds with V.32 training and the appropriate securitysignaling (see FIG. 3B).

[0056] If, at step 224, the responder detects the start of STU-IIIMessage A within the 150 msec window, then the responder determines thatthe far-end modem is a STU-III modem operating in alternate mode. Atstep 226, the responder stops the 150 msec timer, and transmits MessageB. At step 228, the responder completes STU-III alternate mode signaling(see FIG. 3C).

[0057] If, at step 230, the responder determines that it has notdetected either a V.32 “AC” response or a STU-III Message A responsewithin the 150 msec window, then the responder determines that thefar-end modem is a STU-III modem in interoperable mode. At step 232, theresponder transmits the SCR1 and, at step 234, completes STU-IIIinteroperable mode signaling (see FIG. 3D).

[0058] The present invention is possible since a commercial V.seriesmodem is looking for energy at 1800 Hz. Since the transmission of an1800 Hz tone with 45-degree phase reversals would be in violation ofV.32 specifications (which require a pure 1800 Hz tone), the V.seriesinitiator will ignore the 45 degree phase shifts present in a P1800 Hztone, and consider the tone to be the expected “AA” tone. Also, theV.series initiator ignores the phase reversals in the “alternate mode”P1800 Hz tone, which occur in the first 80 msec of reply, since it looksfor phase reversals only after it transmits “AC”, which can occur nosooner than 155 msec after the start of the P1800 Hz tone.

[0059] In a preferred embodiment, an auto-negotiating, multi-mode modemaccording to the present invention can be implemented in a digitalsignal processor (DSP) coupled to a telephone network viaanalog-to-digital (A/D) and digital-to-analog (D/A) converters and atelephone hybrid. The telephone hybrid converts the two-wire interfaceused in analog telephone circuits into separate transmit and receivesignals (i.e., four-wire). These four-wire signals are then convertedbetween the analog domain and the digital domain with A/D (for thereceive path) and D/A (for the transmit path) converters. The resultingdigital representation of the analog signals is processed by a DSP. TheDSP analyzes received signals to determine frequency and phaseinformation, and generates transmit signals of the proper frequency andphase to convey the appropriate signaling. By analyzing and manipulatingthe frequency and phase of the signals, the DSP performs amodulator/demodulator (modem) function.

[0060] To support digital telephone interfaces, like ISDN, the telephonehybrid and D/A and A/D converters are eliminated, and are replaced withan ISDN network interface function. The DSP would then also include acompanding function to interface to the A-law or μ-law coding.

[0061] The methods of the present invention can be also be implementedin software, and stored as computer-executable instructions on a fixedor removable computer-readable medium, such as a microprocessor,computer hard drive, floppy disk, or the like.

[0062] Thus there have been described apparatus and methods fordetermining an operational mode of a far-end modem. Those skilled in theart will appreciate that numerous changes and modifications may be madeto the preferred embodiments of the invention and that such changes andmodifications may be made without departing from the spirit of theinvention. It is therefore intended that the appended claims cover allsuch equivalent variations as fall within the true spirit and scope ofthe invention.

I claim:
 1. A method for identifying a far-end modem type, comprising:transmitting a V.8 ANS_(am) tone to the far-end modem; receiving aresponse signal from the far-end modem in response to the transmittedV.8 ANS_(am) tone; and determining from the response signal whether thefar-end modem is a commercial modem or a secure modem.
 2. The method ofclaim 1, wherein determining whether the far-end modem is a commercialmodem or a secure modem comprises determining whether the far-end modemis a V.series modem or an FSVS modem.
 3. The method of claim 2, whereindetermining whether the far-end modem is a commercial modem or a securemodem comprises: determining whether the response signal is a V.8 CMtone; and if the response signal is a V.8 CM tone, determining that thefar-end modem as a V.8 modem.
 4. The method of claim 1, whereindetermining whether the far-end modem is a commercial modem or a securemodem comprises: determining whether the response signal has a nominalfrequency of about 1800 Hz; and if the response signal has a nominalfrequency of about 1800 Hz, determining from the response signal whetherthe far-end modem is a V.32 modem or a secure modem.
 5. The method ofclaim 4, wherein determining whether the far-end modem is a V.32 modemor a secure modem comprises: determining whether the response signalincludes phase shifts; and if the response signal does not include phaseshifts, determining that the far-end modem is a V.32 modem.
 6. Themethod of claim 4, wherein determining whether the far-end modem is aV.32 modem or a secure modem comprises: determining whether the responsesignal includes phase shifts; and if the response signal includes phaseshifts, determining that the far-end modem is a secure modem.
 7. Themethod of claim 1, further comprising: determining from the responsesignal, an operational mode of the far-end modem.
 8. The method of claim7, wherein determining the operational mode of the far-end modemcomprises: determining whether the response signal includes phasereversals; and if the response signal includes phase reversals,determining that the far-end modem is an FSVS modem in alternate mode.9. The method of claim 7, wherein determining the operational mode ofthe far-end modem comprises: determining whether the response signalincludes a 128 dibit gap; and if the response signal includes a 128dibit gap, determining that the far-end modem is an FSVS modem inhalf-duplex mode.
 10. The method of claim 7, wherein determining theoperational mode of the far-end modem comprises: determining whether theresponse signal includes phase reversals; determining whether theresponse signal includes a 128 dibit gap; and if the response signaldoes not include phase reversals or a 128 dibit gap, determining thatthe far-end modem is an FSVS modem in interoperable mode.
 11. A methodfor determining a far-end modem type, comprising: transmitting to thefar-end modem a P1800 Hz tone with phase reversals; receiving a responsesignal from the far-end modem in response to the transmitted P1800 Hztone; and determining from the response signal whether the far-end modemis a commercial modem or a secure modem.
 12. The method of claim 11,wherein determining from the response signal whether the far-end modemis a commercial modem or a secure modem comprises: determining whetherthe far-end modem is a V.32 modem or a secure modem.
 13. The method ofclaim 12, wherein determining whether the far-end modem is a V.32 modemor a secure modem comprises: determining whether the response signalincludes a V.32 AC; and if the response signal includes a V.32 AC,determining that the far-end modem is a V.32 modem.
 14. The method ofclaim 11, further comprising: determining whether the response signalincludes an FSVS Message A; and if the response signal includes an FSVSMessage A, determining that the far-end modem is an FSVS modem inalternate signaling mode.
 15. The method of claim 11, furthercomprising: determining whether the response signal includes a V.32 AC;determining whether the response signal includes an FSVS Message A; andif the response signal includes neither a V.32 AC nor an FSVS Message A,determining that the far-end modem is an FSVS modem in interoperablemode.
 16. The method of claim 11, further comprising: monitoring anincoming channel for energy at 2100 Hz; and if 2100 Hz energy is presentin the incoming channel for at least about one second, then determiningwhether the far-end modem is a V.32 compliant commercial modem or asecure modem in interoperable mode or alternate mode.
 17. A method fordetermining a far-end modem type, comprising: monitoring an incomingchannel for the presence of any of 1800 Hz energy or 2100 Hz energy; if1800 Hz energy is present in the incoming channel for at least about onesecond, then determining that the far-end modem is a secure modem inhalf-duplex mode; and if 2100 Hz energy is present in the incomingchannel for at least about one second, then determining whether thefar-end modem is a V.32 modem or a secure modem in interoperable mode oralternate mode.
 18. The method of claim 17, further comprising:monitoring the incoming channel for the presence of a V.8 ANS_(am) tone;and if a V.8 ANS_(am) tone is present in the incoming channel, thendetermining that the far-end modem is a V.8 modem.
 19. Acomputer-readable medium having stored thereon computer executableinstructions for performing a method for identifying a far-end modemtype, comprising: transmitting a V.8 ANS_(am) tone to the far-end modem;receiving a response signal from the far-end modem in response to thetransmitted V.8 ANS_(am) tone; and determining from the response signalwhether the far-end modem is a commercial modem or a secure modem.
 20. Acomputer-readable medium having stored thereon computer executableinstructions for performing a method for identifying a far-end modemtype, comprising: transmitting to the far-end modem a P1800 Hz tone withphase reversals; receiving a response signal from the far-end modem inresponse to the transmitted P1800 Hz tone; and determining from theresponse signal whether the far-end modem is a commercial modem or asecure modem.